The 'tan a hide' job is automatically triggered after the game notices that there is a skin to be tanned within a certain distance of the tanner's shop. The player can turn off this behaviour by going into the orders menu, choosing Workshop Orders, and then turning Auto tan off.

If you have turned Auto Tan off, your manager (units, manager) can manually queue "Tan a Hide" jobs. This is also useful if for some reason Auto Tan ignores a skin.